drm/amd/display: Don't use stereo sync and audio on RGB signals (v2)

drm/amd/display: Don't use stereo sync and audio on RGB signals (v2)

Analog video signals on VGA or DVI-A (analog part of DVI-I)
don't support audio, so avoid calling any audio related
functions on analog signals.

Stereo sync was not set up for analog signals in the legacy
display code either, so there is no loss of functionality if
we omit it from DC for now.

Also add a dc_is_rgb_signal similar to other dc_is_*_signal.

v2:
Added comment to clarify what we mean by RGB in this context.

Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>